FIELD PHENOMENA DUE TO MICROSCOPIC ORGANISMS. 9
it is evident from the exposed positions in which it occurs that it
will often be subject to extended periods of drought and also at
times to intense cold. But the liability to desiccation is no
doubt the most critical condition of its environment and the one
to which it has become specially adapted. That it is so adapted
in a very marked degree is quite certain both from the fact of its
survival in the natural positions in which it occurs and from
direct experiments carried out by Prof. Fritsch and others.
When it is remembered that all the Pleurococus cells are in the
vegetative state this tolerance of periodical dry conditions is very
remarkable. It is, however, very difficult in this case to say
exactly to what modification the power of resisting desiccation
is due, for the appearance of the cells themselves does not seem to
throw any light on the matter. It is just possible that the
production of the unicellular condition of the plant is connected
in some way with adaptation to alternating moist and dry
periods, for there are good reasons for supposing that Pleuro-
coccus, as we know it to-day, does indeed represent a retro-
gression from an aquatic multicellular alga to a stage in which
the cells become entirely free from one another or very nearly
so. Its method of reproduction is similar to that found in cells
forming part of multicellular tissue and its occasional formation
of filaments and small plates of tissue must be regarded as remin-
iscent of a past phase rather than as a preliminary attempt to
reach a higher condition in the future, because the effect of similar
conditions upon other sub-aerial algae, which still have close
relatives leading an aquatic existence, is in exactly the same
direction, namely, the disintegration of multicellular tissue into
simpler elements, such as filaments or single cells, as for example
in the species of Stichococcus and Prasiola, to be referred to
presently.
In our common tree-incrusting alga Pleurococcus, therefore,
we probably have a very good example of a plant which, in order
to reach a higher ecological position (for we must. I think, consider
any attempt to live permanently out of the water as a striving
after higher things) has had to forego the advance it had already
made in the direction of cellular combination and revert once
again to the unicellular state.
As already mentioned, Pleurococcus is very frequently accom-
panied by other kinds of algae, usually in much smaller numbers